Common iguana. Credit: Bjørn Christian Tørrissen. (CC BY-SA 4.0)
Common iguana. Credit: Bjørn Christian Tørrissen. (CC BY-SA 4.0)

What Disco Fog Taught Us About Iguana Lungs

Froggys Fog Swamp Juice is billed as “the world’s greatest fog”. According to the manufacturers, it produces a long-lasting artificial mist that has been used in haunted houses, nightclubs, skating rinks, theme parks, and even police and fire departments.

Colleen Farmer used it to study how an iguana breathes.

She threaded an endoscope—a tube with a light and a camera at the end—into the lizard’s nose, while allowing it to inhale the artificial smoke from a fog machine. The smoke, though harmless, contained small particles, and the camera could detect these they worked their way into the iguana’s lungs.

And to Farmer’s surprise, no matter whether the lizard breathed in or out, the smoke particles only moved in one direction.

To understand why that’s weird, consider how your own lungs work. When you inhale, you suck in fresh air so that oxygen can pass into your blood. When you exhale, you expel the stale air back in the same direction. Air moves through your lungs like the tides: in and out, in and out.

A bird’s lungs work very differently, and this image explains it well. Think of them as a radiator with air sacs at either end. As the bird inhales, it draws air into the rear air sacs. It exhales, and this air is forced through the radiator-like lungs, where oxygen passes into the blood. The next inhalation drives the stale air into the front air sacs, and the next exhalation drives it out of the bird. It takes two breaths for any batch of air to circulate through the bird’s body. More importantly, whether the bird is inhaling or exhaling, air only ever flows one way through its lungs,.

All of this was discovered in the 1970s. During the following decades, biologists thought that this set-up was unique to birds. It presumably helped them to extract as much oxygen as possible from their breaths, to fuel their high-octane, warm-blooded, fast-flying lifestyle.

“Then in 2010,” as Matt Wedel wrote on his blog, “Colleen Farmer and Kent Sanders of the University of Utah blew our collective minds by demonstrating that alligators have unidirectional flow-through lungs, too.” Alligators aren’t warm-blooded and, last I checked, they can’t fly. That was a strong hint that one-way lungs evolved for different reasons than the ones people had assumed. The discovery also meant that these lungs probably evolved in the common ancestor of the archosaurs—the group of reptiles that includes birds, crocodiles, all the extinct dinosaurs, and pterosaurs.

But Farmer wasn’t finished. Last year, her team showed that a lizard—the savannah monitoralso has one-way lungs. Again, everything changed. Monitors are fast hunters that can run down mammalian prey; perhaps that’s because of their lungs. They aren’t archosaurs either. They belong to a different group of reptiles called the lepidosaurs, which includes all snakes and lizards. So, either the archosaurs and lepidosaurs evolved one-way lungs independently, or the set-up actually existed in their common ancestor and is much older than anyone thought.

That brings us back to the iguana and the Froggys Fog Swamp Juice. After the monitor lizard discovery, Farmer wanted to study another lepidosaur, and one that doesn’t have the same energetic lifestyle. The common iguanas were perfect. They can actually sprint faster than monitors, but a weird anatomical quirk cuts the blood flow to their legs when they try. They soon have to stop because they build up too much lactic acid. They have no stamina.

Their lungs are also incredibly simple. There are no complicated sacs or radiators like in birds, alligators or monitors. Each lung consists of just two chambers. Air enters and leaves each of these through a single hole. Aside from the holes, the chambers are completely airtight, with no bridges between that. Farmer’s team, including Robert Cieri and Emma Schachner, proved this by pumping air into the front chamber of a surgically removed lung and sealing the hole with latex. They then put the lung underwater and tried to squeeze the air into the back chamber. No dice.

“No one in their right mind would think that there’s unidirectional flow in these lungs,” says Farmer. “If you were an engineer and tried to come up with a system with one-way flow, this is not what you’d think of.” And yet, one-way flow is exactly what her team found.

They studied the iguanas using several techniques. They watched synthetic smoke travel around the lizards’ lungs as they breathed normally. They implanted airflow meters in the animals. They pumped water full of pollen grains through surgically removed lungs, and watched the flow of the particles. And their colleague Brent Craven created a computer model that simulated the flow of air through virtual iguana lungs.

All of these techniques gave the same results. Air enters each chamber at high speed and jets straight to the back. It then branches off to the side, hugging the chamber walls as it moves back to the front. Eventually, it leaves via the same hole it entered. This means that the central part of each chamber is tidal—air moves in and out as the animal breathes. But along the walls of the chamber, air only ever moves in a single back-to-front direction. And the walls are where the blood vessels are—they’re the places where oxygen moves from the lungs into the bloodstream.

So, even though the iguana’s lung is incredibly simple, it achieves the same results as a hawk, crocodile, or Komodo dragon. At every part of its breathing cycle, its blood receives a steady stream of fresh oxygen.

“Nobody’s going to argue that this enables the iguana to be an endurance athlete,” says Farmer. Instead, she speculates that one-way lungs evolved because they allowed their owners to hold their breaths for long periods of time. That ability would have been especially useful to animals that hide from predators by blending into the background. Any movement would give them away, and breathing creates movement. If the animal holds its breath, its beating heart can still push air through its one-way lungs, allowing it to extract as much oxygen as possible.

These early one-way lungs probably worked like those of the iguana. Their owners could then have expanded on this simple structure by adding dividing walls in the centre of the lung where air flows tidally. Now, instead of a simple chamber, you have a series of connected sacs, as in birds and alligators. And these more efficient lungs allowed some groups like the birds and monitors to explore a more active existence.

“We’d expect to see better-developed unidirectional flow in species that rely heavily on crypsis,” she says, “whereas animals that are poisonous wouldn’t care.” She plans on testing this idea by studying the lungs of more reptiles. Chameleons, for example, rely on camouflage and have huge lungs. Do these organs have one-way flow? Farmer wants to find out.

But first, she has her eye set on a different target—the tuatara. This New Zealand resident looks like a lizard, but isn’t. Instead, it’s the only survivor of a largely extinct group of lepidosaurs, one that’s separate from the snakes and lizards. If it also has a one-way lung, that would really strengthen the case that such a structure was present in the ancestor of all living reptiles.

Reference: Cieria, Craven, Schachner & Farmer. 2014. New insight into the evolution of the vertebrate respiratory system and the discovery of unidirectional airflow in iguana lungs. PNAS